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A330 ZFW/ZFCG DISAGREE

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A330 ZFW/ZFCG DISAGREE

Old 17th Jan 2013, 21:02
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I remember coming across this a long, long time ago. If memory serves me right, isn’t it the case that the FCMC used the pilot-entered ZFWCG and calculates the GWCG by taking the actual fuel mass/distribution onboard on a per-tank basis, whereas the FMGC used the pilot-entered ZFWCG and calculates the GWCG by assuming a standard fuel distribution for any given FOB? A non-standard fuel distribution would, of course, result in a different fuel CG effect and hence, a difference between the GWCG outputs from the FCMC and FMGC.



Also, had you already entered a BLOCK figure in INIT B when the message appeared?
It slowly starting making some sense
Normally the BLOCK fuel is already there. Then we enter ZFW/ZFWCG and finally get the message.
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Old 17th Jan 2013, 21:50
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Normally the BLOCK fuel is already there. Then we enter ZFW/ZFWCG and finally
get the message.
The time it happened to me, we had done exactly the same.

I'm fairly confident that it's something to do with the difference between the actual FOB and the pilot-entered BLOCK figure in INIT B, and the consequent difference in the result of a particular calculation that the FCMC and FMGC each do. I just can't, for my life, remember the precise definition.

One thing that doesn't help is the lack of (clear) information from Airbus regarding this - it's just a case of us knowing the logic that the computers use and what is done with what figure. Cue the discussion that the airplane is a flying computer program.

Last edited by Old Grouch; 17th Jan 2013 at 22:04.
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Old 17th Jan 2013, 23:17
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1/ loud "ding" ECAM. Scares ya, now read the ECAM.

FCOM 3 says the message:-----


FUEL ZFW ZFWCG DISAGREE

This caution is triggered in case of disagree between ZFW or ZFCG values from FMGC 1 and 2.
– FMGC VALUES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONFIRM Confirm that the ZFW and ZFCG values from each FMGC are the same as the loadsheet values



2/ both crew double check their respective CDU's contain correct info as per loadsheet.
3/ Breath
4/ move along........these things happen.

Last edited by nitpicker330; 17th Jan 2013 at 23:26.
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Old 18th Jan 2013, 09:05
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The way I look at it, there is no ZFW nor ZFCG computation. They are always pilot entered. There is no way that those values differ from FMGC computed ones because the FMGC does mot compute any ZF values.

The FMGC only computes GW values (and TO and LDG) bases on ZF and block fuel pilot entered inputs

Now, if there is a difference between FCMC computed GW and/or GWCG (which use pilot entered ZFW, ZFCG and FCMC sensed fuel quantity) and FMGEC's (which also use pilot entered ZFW & ZFCG)... Why does it trigger a ZFW/ZFCG discrepancy? It is clearly a fuel related discrepancy, that of pilot entered block and sensed quantity, and even the ECAM is a FUEL one.

Is there any ECAM alerting from a difference between FCMC and FMC fuel quantity?
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Old 18th Jan 2013, 10:16
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The way I look at it, there is no ZFW nor ZFCG computation. They are
always pilot entered.
Absolutely correct. Perhaps I wasn't clear earlier. When I said "...and the consequent difference in the result of a particular calculation that the FCMCs and FMGCs each do", I meant a calculation that uses ZFW/ZFWCG values to derive another value (the GW/GWCG), not a calculation that reults in in the ZFW/ZFWCG. The ZFW/ZFWCG are indeed always pilot-entered and, of course, are the starting point of the calculations.

It is clearly a fuel related discrepancy
I quite agree. Incidentally, when we got this message, like Romasik says, we had indeed not uplifted any fuel on a turnaround.

We can apply the relevant procedure, but without knowing the 'thought process' of the system behind the message, it's difficult to gain a full understanding of it and its background.
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Old 18th Jan 2013, 10:46
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Thank you for the additional info, A33Zab. For the record, we did not do an autoland prior to having the message.
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Old 18th Jan 2013, 12:22
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Hmmm

Quite complicated isaue, isn't it

Is pilot entered block fuel ever used for any computation?
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Old 18th Jan 2013, 13:44
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Is pilot entered block fuel ever used for any computation?
I no longer have my A330 manuals and I can't seem to find the full set online. I don't want to rely on memory and potentially also mix it up with any A320-specific information that I was also on, so I wouldn't like to give a precise answer, which would be conter-productive, if wrong. In other words, take what I say with a pinch of salt.

Anyway, from what I could find/remember:

"The FMGC computes the FPLN predictions, based on the entered BLOCK fuel, and estimates the EXTRA fuel value." - this is before engine start.

"After engine start, the FMGC will stop using the pilot-entered block fuel and will compute its predictions based on the FOB indicated by the FCMC"

Perhaps the same before/after-engine-start logic applies to the FMGC GW/GWCG calculation? I.e. the BLOCK value is used in the FMGC GW/GWCG calculation before engine start? If so, a difference of sufficient mangitude between the actual FOB and the BLOCK values would result in differing FCMC and FMGC GW/GWCG values and hence the message.

Apoligies if I am talking rubbish here.

EDIT: Perhaps Mr DozyWannabe has some info?

Last edited by Old Grouch; 18th Jan 2013 at 13:50.
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Old 18th Jan 2013, 17:04
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Alas I don't have any specifics on this system right now.
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Old 18th Jan 2013, 18:03
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Regarding the fuel figures. We always enter the exact value displayed on ECAM (well, maximum +/- 50 kg) and still get the message...
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Old 18th Jan 2013, 18:26
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Regarding the fuel figures. We always enter the exact value displayed on ECAM (well, maximum +/- 50 kg) and still get the message...
So more or less back to square one.

Alas I don't have any specifics on this system right now.
Then your quest awaits, Sir!

Joking aside and rather off-topic, this reminds me a lot of the 'CG DISAG (FUEL)' message that one could get on the MD11. Similarly to the Airbuses, the setup here was that the loadsheet ZFW/ZFWCG values are entered in the WEIGHT INIT page (the MD11 equivalent of INIT B. Infact, the MD11 CDU interface is a lot like the Airbus MCDU one, albeit rather more basic, e.g. no colours) and are then used by the FSC (Fuel System Controller) to calculate and display the GW/GWCG on the SD.

The important difference, of course, is that in addition to the ZFW/ZFWCG, the loadsheet TOW/TOCG are also entered in the FMS. Now, if one entered wrong TOW/TOCG values, which were sufficiently different to the FSC-derived GW/GWCG (there is some tolerance; to account for taxi fuel, for one), after start-up, one would get the aforementioned 'CG DISAG (FUEL)' message, defined as:

CG DISAG (FUEL) - Disagreement between the aircraft CG displayed on the SD and the CG entered in the FMS. Confirm fuel load and entered data.

This would happen more often if doing a manual loadsheet (ours were drop-line type), as the 'CG change due to fuel' measurement on a manual loadsheet is, ofcourse, less precise than that of a computer loadsheet or the FSC calculation.

So in summary of the above, on the MD11, there is a clear opportunity for a weight/CG discrepancy. Where is discrepancy arising on the A330?
Perhaps it's just a case of a momentary lapse of communication between the FMGECs/FCMCs? Or a case of a GWCG based on an assumed/standard fuel distribution in the FMGECs vs actual/non-standard fuel distribution in the FCMCs?

Uplinker, I got your post in a notification email, but it seems that you've deleted it from the forum?

Last edited by Old Grouch; 18th Jan 2013 at 18:32.
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Old 19th Jan 2013, 11:13
  #52 (permalink)  
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Where is discrepancy arising on the A330?
... Or a case of a GWCG based on an assumed/standard fuel distribution in the FMGECs vs actual/non-standard fuel distribution in the FCMCs?
Looks like having the most sense.
As an additional factor it could also be a case of french english. They should have hired a team of native speakers to take part in creating/maintaining aircraft/human interface. I never thought that reading simple phrases in english english, like:
CG DISAG (FUEL) - Disagreement between the aircraft CG displayed on the SD and the CG entered in the FMS. Confirm fuel load and entered data.
would give so much pleasure compare to the respective Airbus FCOM phrase
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Old 19th Jan 2013, 11:32
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That's all certainly true. The writing style and logic of the FCOMs certainly has its distinct 'flavour'.
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Old 20th Jan 2013, 15:00
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Here we are. Got it from Aeroflot pilot's forum:

Description
1) What are the conditions to trigger the "FUEL: ZFW/ZFCG DISAGREE" message?

2) How can be triggered a spurious "FUEL: ZFW/ZFCG DISAGREE" message?

Solution
1) The conditions for the FCMC to trigger the ECAM warning "FUEL: ZFW/ZFCG DISAGREE" are the following:
1- the value of ZFW sent by the FMS differs between side 1 and side 2 of more than 100 lbs,
or
2- the value of ZFWCG sent by the FMS differs between side 1 and side 2 of more than 0.025 %,
or
3- during the initialization/re-initialization process, the status of ZFW/ZFWCG sent by the FMS differs between side 1 and side 2 during more than 2 seconds.

2) Investigation of the spurious cases of ECAM warnings "FUEL: ZFW/ZFCG DISAGREE" on A330/A340 basic aircrafts showed that when a ZFW/ZFWCG is inserted on one MCDU, each FMS will send these values to their onside FCMC with a boolean set to high (to indicate to the FCMC to init/re-init the values) during a few seconds. But, the setting/resetting of the high position for this boolean is not synchronized between both FMSs. A delta of more than 4 s may be observed.
And because of the 3rd condition, the FCMC issues the message.

This FMS/FCMC interface issue is addressed in the FMS "Release 1A" (R1A) standard.
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Old 20th Jan 2013, 15:21
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Uplinker, I got your post in a notification email, but it seems that you've deleted it from the forum?
Hi Old Grouch. Not intentionally; My PC, or the Pprune site were doing really weird things to my attempted posts the other day - putting them in the middle of the thread instead of at the end etc. If you can, please would you repost my post?

Last edited by Uplinker; 20th Jan 2013 at 15:21.
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Old 20th Jan 2013, 15:39
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Investigation of the spurious cases of ECAM warnings "FUEL: ZFW/ZFCG
DISAGREE" on A330/A340 basic aircrafts showed that when a ZFW/ZFWCG is inserted
on one MCDU, each FMS will send these values to their onside FCMC with a boolean
set to high (to indicate to the FCMC to init/re-init the values) during a few
seconds. But, the setting/resetting of the high position for this boolean is not
synchronized between both FMSs. A delta of more than 4 s may be observed.
And
because of the 3rd condition, the FCMC issues the message.
Excellent, thank you for that. So something akin to "a momentary lapse of communication".

Hi Old Grouch. Not intentionally; My PC, or the Pprune site were doing really
weird things to my attempted posts the other day - putting them in the middle of
the thread instead of at the end etc. If you can, please would you repost my
post?
Certainly, here we go:

Yes; pilot entered block fuel is used to make initial calculations according to the flight plan and winds.

This gives a gross error check; that the FMGEC (A330) agrees with the paper flight plan that fuel on board will be sufficient, given the winds, to complete the flight with sufficient reserve.

Later after engine start, the system refines its predictions based on actual fuel on board.

In our company we enter an estimated ZFW/ZFWCG after inputting the flight plan and winds. I've had the ECAM caution in question when taxying away after a turnaround in which we uploaded fuel, and after engine start (obviously). But ONLY when we have done a manual load sheet. I am trying to remember what we do differently with data inputs when we do a manual loadsheet. Perhaps we enter the estimates on one MCDU and the actuals on the other? Then if the FMGEC's don't talk to each other properly, we get the discrepancy and the caution?

Thank you, everybody who contributed.

Last edited by Old Grouch; 20th Jan 2013 at 15:47.
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Old 20th Jan 2013, 15:41
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FMS

2- the value of ZFWCG sent by the FMS differs between side 1 and side 2 of more than 0.025 %

For me this is the common reason that is why I do not pay too much attention to this software glitch
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Old 20th Jan 2013, 19:40
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However, the FCOM DES FUEL ECAM warnings says that the caution is triggered by discrepancy between FMC and FCMC, not between FMC1 and 2... Why is it all so confusing??
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Old 20th Jan 2013, 20:44
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The way I understood/presumed it to be (not being a software engineer, it is pure speculation on my part, apologies if it is rubbish), is that it comes down to this:

during the initialization/re-initialization process, the status of ZFW/ZFWCG sent by the FMS differs between side 1 and side 2 during more than 2 seconds.
But, the setting/resetting of the high position for this boolean is not synchronized between both FMSs
So, let's take it step-by-step:
-Presumably, prior to a ZFW/CG entry in INIT B, each FCMC stores either a blank or a '0' value for this data. Let's call this value 'x'

-A 'boolean' setting tells the FCMCs whether ZFW/CG data (let's call this ZFW/CG value 'y') from the FMGCs is being sent to them for their initialisation. If this 'boolean' setting is signalling an initialisation, the respective FCMC accepts it as given and stores the new value.

-If the 'boolean' status is not signalling an initialisation, because, according to the quote above, it is not synchronised between the FMGCs, (the following is pure speculation on my part) as a safeguard, the respective FCMC does not accept the new value, e.g. because it could be a spurious/uncommanded signal.

-So, in one of the FCMCs, the previously-stored value 'x' does not equal the newly-sent, but rejected, FMGC value 'y', which triggers the message.

Last edited by Old Grouch; 21st Jan 2013 at 20:59.
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Old 21st Jan 2013, 18:36
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Originally Posted by Magnetic Iron
For me this is the common reason that is why I do not pay too much attention to this software glitch
Judging by the info Old Grouch unearthed, it's not a glitch so much as a redundancy safety feature that sometimes gets triggered with real-world values.

@OG - As a software engineer, I think it looks like you've got a good, succinct description there - not rubbish at all!
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